1. Field of the Invention
The present invention relates to a vehicle height adjusting device, and more particularly to a vehicle equipped with an air suspension system.
2. Description of the Related Art
A bus, which is used herein to represent a conventional air suspension equipped vehicle, as depicted in FIG. 16, has a bus body 1 supported by a frame 1a around a front suspension, a front axle 2a, and air springs 3a which are interposed between the front frame 1a and the front axle 2a; and a frame 1b around a rear suspension, a rear axle 2b which includes an axle case and an air-spring support beam that is fixed to the axle case, and air springs 3b which are interposed between the rear frame 2b and the rear axle 2b.
Such an air suspension equipped vehicle generally has an air suspension circuit such as shown in FIG. 17. In the illustration, reference numeral 4 denotes an air compressor that is mounted, for example, on an engine (not shown); reference numeral 5 denotes a main tank that stores compressed air supplied by the compressor 4 via an air pipe 6a, which has an air filter 15, a pressure regulating valve 14 and a check valve 16 arranged therein; reference numerals 6, 6c and 6d denote air pipes that carry the compressed air from the main tank 5 to air equipments (not shown) and form the suspension air circuit; and reference numerals 7a and 7b denote well-known leveling valves which are fixed to the bus body and whose respective components include levers 7a1 and 7b1, the ends of which are rotatably supported by the body 1 while the other ends of which are linked to the axles 2a and 2b, respectively. When the pivotal movement of the levers 7a1 and 7b1 detects that the relative positions of the axles 2a and 2b are nearer than their reference positions (i.e., the vehicle body sinks), the leveling valves 7a and 7b permit air to flow from the main tank 5 through the air pipes 6, 6c and 6d to the air springs 3a and 3b. Conversely, when the relative positions of the axles 2a and 2b are farther than their reference positions (i.e., the vehicle body floats), the leveling valves 7a and 7b cut off the air flow from the main tank 5, and exhaust the air within the air springs 3a and 3b to lower the body 1. It should be noted that the leveling valves 7a and 7b operate independently for the front and rear parts of the body, and that they are closed to inhibit the flow of air through the air pipes 6, 6c and 6d so that the air springs 3a and 3b may perform as simple springs for small vibrations encountered under normal running conditions.
This bus suspension system, as shown in FIG. 16, is controlled to maintain a constant vehicle height by using the leveling valves 7a and 7b and therefore an approach angle .alpha., a departure angle .beta., and a road clearance m are ensured to prevent the vehicle body from touching a road surface while running. On the other hand, the lower the step heights 1 and 1' in FIG. 16 the more convenient for passengers when getting on and off, and time required therefor will be reduced. The reduction of the step heights 1 and 1', however, conflicts with ensuring sufficient values of .alpha., .beta., and m.
To resolve this problem of conflicting requirements, a function called kneeling has been studied, and many kneeling systems have been proposed. As one example of these systems, Japanese Unexamined Utility Model Publication No. Sho 48-5423, as shown in FIG. 18, has additional solenoid (or manual) three-way switching valves 8a and 8b which are disposed between the leveling valves 7a and 7b and the air springs 3a and 3b, respectively.
This arrangement, under normal conditions, makes the solenoid valve 8a and 8b inactive, thereby communicating the air springs 3a and 3b with the respective leveling valves 7a and 7b to maintain a constant vehicle height. When the vehicle height is lowered, the solenoid valves 8a and 8b are made active (or are manually switched over), and their valve positions are switched over to cut off the air flow between the air springs 3a and 3b and the leveling valves 7a and 7b, while the heights H of the air springs 3a and 3b are decreased by exhausting the air within the air springs 3a and 3b to the atmosphere.
To increase the vehicle height, i.e., to return the vehicle to its original height, the solenoid valves 8a and 8b are made inactive. The air from the main tank 5 is then fed through the leveling valves 7a and 7b to the air springs 3a and 3b until the arms of the leveling valves 7a and 7b become almost horizontal, resulting in the reference height H.
As described above, a kneeling mechanism is designed to facilitate passengers to get on/off and to reduce the time required therefor. The kneeling mechanism, however, requires a great amount of time to return a vehicle to its original height after air has been exhausted. This is because such a vehicle height return is realized by supplying air equal to that exhausted from the solenoid valves 8a and 8b through the main tank 5 from the compressor (not shown), however, with the discharge capacity of a vehicular-mounted compressor, it generally takes several minutes to attain the original vehicle height. Further, since the pressure in the main tank 5 is reduced when the vehicle has returned to its original height, there remains insufficient pressure to drive various equipments.
To compensate for these shortcomings, as illustrated in FIG. 18, one or more auxiliary tanks 9 are provided in which by feeding the air retained in an auxiliary tank 9 to the air springs 3a and 3b the time required to return the vehicle to the original height is reduced.
Although the more a kneeling mechanism with only one auxiliary tank 9 can reduce the time required for attaining the original vehicle height the more the number is increased, an air compressor must be operated continuously for a long time in order to refill the auxiliary tanks that has already released air into the air springs, and the heat built up within the compressor can cause the deterioration of the durability of the air compressor per se.
These shortcomings take advantage of the fact that in pneumatic equipment air as hydraulic fluid can be easily obtained anywhere; and used air can be easily disposed of anywhere. However, whether or not a large volume of air can be provided for a short time depends entirely on an air compressor. To reduce the dependency on an air compressor, there has been proposed a system that does not rely on the above described advantage, i.e., a system that minimizes air exhaustion, or does not exhaust.
As prior art, in the apparatus disclosed in the Japanese Unexamined Patent Publication No. Sho 54-47229, as illustrated in FIG. 19, a supporting leg 30 which raises a bus body 1 behind the center of gravity of the body is provided. A front (Ft) air spring 31 communicates with a rear (Rr) air spring 32 via an air pipe 34 that can carry air from the Ft air spring 31 to the Rr air spring 32.
When, however, the supporting leg 30 is located between the center of gravity and the rear wheels as illustrated, the load supported by the front and rear wheels is reduced by the load supported by the supporting leg 30, so that the front part of the vehicle floats and the vehicle height increases.
Further, even if the supporting leg 30 is located behind the rear wheels to increase the load supported by the front wheels, unless the internal pressure of the Ft air spring 31 exceeds that of the Rr air spring 32, the air flows inversely from the Rr air spring 32, so that the front part of the vehicle floats, and the vehicle height increases.
The supporting leg 30, therefore, must have enough stroke that the internal volume of the Rr air spring 32 can be increased (i.e., expand the air spring) so that the internal pressure of the Rr air spring 32 may always be lower than that of the Ft air spring 31. More specifically, to expand the air spring, it is advantageous to locate the supporting leg closely behind the center of gravity, while to increase the load supported by the Ft air spring 31, it is advantageous to locate the supporting leg behind the rear wheels. In addition to this problem, since passengers' distribution in the vehicle affects the shift of load share, the degree and the time of kneeling may not be constant.
Moreover, in the apparatus disclosed in the above described Japanese Unexamined Patent Publication No. Sho 54-47229, a method is proposed for taking air out of the Ft air spring 31 and feeding it to the Rr air spring 32 by providing a pump 33 that is different from an engine mounted compressor, in an air pipe 34 which directly communicates the Ft air spring 31 with the Rr air spring 32 as illustrated. According to the explanation, this method can overcome a pressure loss resistance (proportional to the air flow rate squared) while the bus is quickly inclined using the supporting leg 30, and depending on the capability of the pump 33, the vehicle can be easily forward inclined without using the supporting leg 30 which is difficult to be adjusted.
However, such prior art where the pump 33 is located in the air pipe 34 directly interconnecting the Ft and Rr air springs 31 and 32, can perform only forward inclinatory kneeling but cannot do kneeling for a middle door section 1d and a rear door section le shown in FIG. 16.
Further, when the vehicle is returned to the horizontal condition, the air put back from the Rr air spring 32 must pass through the pump 33 to the Ft air spring 31. Though this could serve as an air flow control, the suspended weights and the weight share are not uniform, and this may cause a problem in the control and adjustment. Another shortcoming is that a device which can disengage the pump from its driving source even in the case of idling.
An air spring device (moving device) disclosed in Japanese Unexamined Utility Model Publication No. Sho 56-141806 as an example of a device that suppresses or prevents air exhaustion has, as illustrated in FIG. 20, an air reservoir device which comprises an air tank 41, a movable partition member 42 that defines two internal spaces of the air tank 41, and a moving device for the partition member 42 that includes a control motor 46, a worm 45, a worm wheel 44, and a ball screw 43 fixed to the partition member 32. An air chamber 41a that is formed of the air tank 41 and the partition member 42 is connected to a Ft air spring 3a (or a Rr air spring 3b) by an air pipe 6c (or 6d), where the internal volumes of the air chamber 41a, the Ft air spring 3a (or the Rr air spring 3b), and the air pipe 6c (or 6d) are arranged to be always constant.
As the control motor 46 of this device is rotated, the worm 45 fixed to the shaft (not shown) of the control motor 46 is rotated. Then, the worm wheel 44 which is engaged with the worm 45 is also rotated, and as a consequence of the rotation, the ball screw 43 whose threads connect with the internal threads in the center hole of the worm wheel 44, moves the partition member 42 to the left or right in the diagram. This movement of the partition member 42 increases or decreases the capacity of the air chamber 41a.
More specifically, if this prior art is applied to a kneeling mechanism, the moving device will pull the partition member 42 to the right in the diagram thereby to increase the capacity of the air chamber 41a, whereby the air from the Ft air spring 3a (or the Rr air spring 3b), which is connected to the air chamber 41a, will be carried to and retained in the air chamber 41a without exhausting the air into the atmosphere, and the air spring height H will be reduced. This is because, as described above, the internal volumes of the air chamber 41a, the Ft air spring 3a (or the Rr air spring 3b), and the air pipe 6c (or 6d) are arranged to be always constant. For the same reason, in the return to the original vehicle height, the moving device drives the partition member 42 inversely, i.e., to the left in the diagram, thereby decreasing the capacity of the air chamber 41a, whereby the air retained in the air chamber 41a can be restored in the air spring and the air spring can be returned to its original height H.
However, the control motor 46 that is used in the moving device disclosed in the above described Japanese Unexamined Utility Model Publication No. Sho 56-141806 must support the suspended weight of a heavy commercial bus and control the up-and-down movement of the vehicle. Therefore, it is disadvantageous that the control motor 46 must be a large motor having a great output and requires a generator and a battery that can continuously supply sufficient power to drive such a large motor. Furthermore, the control motor employed must have positioning accuracy and controllability, and quick responsiveness, and must be able to withstand high loads in the presence of an impact input.